There are six models of the Webley No. 1 .455 revolver, Marks I through VI.

There are also six different .455 revolver rounds that were created for the Webley No. 1, Marks I through VI.

There is really no correlation between when the revolvers were manufactured and when the different revolver rounds were created.

I just recently came across a nice compact chart that helps clarify the difference between the revolvers and the ammo with regard to the English "Mark" system. We're gonna need to re-create some of the info that was lost on the old board so I'll throw this out as a first effort.

For the Webley MK I, the Black Powder charge is 18 grains. The Cordite is 11 grains. What this equates to in a modern smokeless powder I have no idea. I asked Alliant some time ago and they told me they didn't have any data to cross reference cordite with any of their powders.

David
"We are not interested in the possibilities of defeat; they do not exist"
Victoria 1899

The loads you cite are for a Mark I case, aren't they? So... which do you have on hand to work with, Mark I or Mk II cases?

I'd love to find some virgin Mk I Dominion cases but I've yet to locate a source. And, Starline has been promising me for two years that they intend to offer Mark II cases, but I'm no longer holding my breath.

In any event, forget Alliant powders, IMHO. Trail Boss loads for Cowboy Action shooting is the smokeless powder that will best duplicate black powder pressures with the highest velocities. And again, IMHO, when manufacturing smokeless loads for guns originally designed for black powder, one is wise to try and duplicate the pressure while living with whatever velocity that load generates.

Typically one encounters two types of black powder guns. The weaker ones use either cast iron or brass frames and should probably be held to pressures of 8,000 psi or less. The better BP guns like the Webley Mk I have steel frames, but the quality of the steel frames used in the BP era is nowhere near as strong as modern steel. Therefore one is wisely cautioned to stay at or under 11,000 psi for these guns. It is absolutely true that many Webley Mk I's have been converted to .45 acp and fired repeatedly with 16,000 psi loads, but that doesn't mean it's a good idea. They typically don't blow up but they undergo accelerated wear because they just weren't designed for modern smokeless pressures.

There are no published data for .455 Trail Boss loads, but you can use some cautious comparison data. For example, the case capacity of the Mk I case is similar enough to the .45 S&W Schofield that you should be able to use the published starting Schofield load of 3.5 grains of Trail Boss with a 250 grain lead bullet. Likewise, the Mk II case capacity is similar enough to .44 Russian that you can use 3.0 grains of TB behind a 250 grain lead bullet.

The pressures generated by these loads will likely be below 9,000 psi, well within the original strength specs for a Mk I revolver. The downside is that velocities are likely to be in the low-to-mid 500 fps range. I have never used the powder charge for the Mk I case as I have none to experiment with but I have used the 3.0 grain data for the Mk II case. My chronograph reports an average velocity of 540 fps. I can tell you with authority that tin cans and paper targets are unable to tell the difference between this light load and a full-bore 16,000 psi .45 acp load.

One thing I did do though was to switch to 230 grain bullets, and that brought the velocities into the low 600 fps range. They also shot closer to point of aim.